This application relates to the art of vehicle air brake systems and valves used therein. More specifically, the invention relates to an improved pressure holding valve for maintaining spring set parking brakes in a released condition even though the air brake system is at relatively low operating pressures. Although the improved pressure holding valve of the present application is particularly applicable for use in an air brake system and will be described with reference thereto, it will be appreciated that the valve can be used in other systems where it is desired to prevent backflow until a predetermined pressure differential develops.
Known vehicle air brake systems include spring set air released parking brakes. One system including a parking brake of this type is disclosed in U.S. Pat. No. 3,285,627 issued Nov. 15, 1966, to Avrea. In systems of this type, release of the parking brakes requires a predetermined pressure of around 80 psig. The air brake system itself is normally operated at a pressure of around 125 psig. During stop and go driving or other heavy brake usage periods, it is possible for the air supply to be depleted faster than the compressor can recharge it. Therefore, the brake system may be at operational pressures between 30-80 psig where the vehicle service brakes can still be operated. At these relatively low operational pressures, the air in the parking brakes would normally bleed down to the same operational pressure. When the air in the parking brakes is at a pressure less than 80 psig, the parking brake is partially applied. As the air pressure is further depleted, the parking brake is applied with greater force under the spring bias. It would be desirable to have a system in which the parking brakes would not drag at relatively low operational pressures of the air brake system.